Transport processes play an important role in nitrogen allocation in higher plants. Amino acids circulate through the vascular system with multiple possibilities for carrier-mediated interchange between phloem and xylem. There is a longstanding debate on the number and specificity of transport systems involved. To dissect the apparent complexity of amino acid transport at the physiological level, respective transporter genes have been isolated by complementation of yeast transport mutants.
An amino acid transporter from Arabidopsis thaliana was identified which is a high-affinity transporter for both lysine and histidine (Chen and Bush (1997) Plant Physiol. 115:1127-1134). This transporter (LHT1) is an integral membrane protein and has little affinity for arginine. LHT1 is present in all tissues, with strongest expression in young leaves, flowers, and siliques. A corn EST (NCBI General Identifier No. 5510817) and four rice ESTs (NCBI General Identifier Nos. 4716084, 2310110, 4716501 and 4716085) have sequence similarity with this Arabidopsis thaliana protein.
Under stress conditions, massive changes in partitioning of carbon and nitrogen take place. In transgenic tobacco overproduction of proline results in decreased tolerance to osmotic stress (Verma and Hong (1996) Plant Physiol. 110:1051-1053). Transforming a yeast mutant defective in amino acid transport with an Arabidopsis thaliana cDNA library, and selecting transformants on medium containing proline led to the identification of specific proline transporters (Rentsch, D. et al. (1996) Plant Cell 8:1437-1446). The two proline transporters (ProT1, and ProT2) are similar to each other and distantly related to amino acid permeases. ProT1 and ProT2 were found expressed in all organs analyzed, with higher levels of ProT1 mRNA being detected in root stems, and flowers. When plants are transferred to a solution containing 200 mM NaCl, ProT2 mRNA accumulated starting at 4 hours after initiation of treatment and increased with time.